I have my concerns about DA but I have to say, I don't just think you're wrong I think you're incredibly wrong.
Even on his bad nights he's one of the best big men we've ever had and we've searched forever for a big man like him that can play both sides of the ball. Secondly, he has stepped up in big moments and while I'd like consistency from him nightly, I'll take a guy who shines in big moments over any "solid defensive big" in the game today.
I'm not in any way equating these two but I remember similar complaints about Shaq after they won their first championship. All too often he'd mail it in on defense and he'd miss a fair amount of games too but when it counted he was the most dominant force in the game. DA is not that force but he is still a matchup nightmare for the rest of the league when he's focused.
If we decide to trade him, I think we'll regret it for a few million years unless our return nets us a young two way star big man that other teams HAVE to account for every night. He's so frustrating but it's the gap between his good nights and bad nights that has us on edge with him. But most of his bad nights are still better than most centers in the game.